Method for the production of zinc sheets particularly adapted for use in galvanic cells



Patented Feb. 20, 1934 ATENT OFFICE METHOD FOR THE PRODUCTION OF ZINCSHEETS PARTICULARLY ADAPTED FOR USE IN GALVANIC CE LLS Paul Mittmann,Ludenscheid in Westfalen, Germany, assignor to New York, N. Y., a

American Lurgi Corporation,

corporation of New York No Drawing. Application April 15, 1931. SerialNo. 530,443, and in Germany April 19, 1930 6 Claims.

This invention relates to rolled sheet zinc, especially for cell casesand like electrotechnical purposes and to a method of manufacturingsame.

Sheet zinc, such as is used for galvanic cells, for example, in theknown cup or sleeve form, for anode batteries, must possess certainpowers of resistance to the action of electrolytes, such as sulphuricacid. Such resistance is necessary, in order to assure sufficientdurability of the finished, but unused cells, in storage, and for theuni orm maintenance of the electromotive force. The testing of sheetzinc by the Cohen method has been elaborated on the basis of theseconsiderations. Said test consists in immersing a plate of a givensuperficial area (25 sq. cm.) in a prescribed volume (100 cc.) of 10%sulphuric acid, and measuring the rise of temperature in the acid, whichshould exceed the initial temperature by 11-13" C. If the temperaturehas not risen above 30 C. in 30 minutes, the zinc is consideredsatisfactory. In reality, the Cohen test fully represents therequirements expected in practice with regard to the suitability of thesheet zinc for galvanic cells.

It has transpired in practice that no possibility existed up to thepresent, of producing sheet zinc in a form which always proved reliableunder the Cohen test. On the contrary, it was found that though some ofthe sheets, from a parcel fairly satisfied requirements, the majorportion of them always gave unsatisfactory results, and therefore couldnot be used at all, or at any rate, not for high-class articles. Thisbehaviour of sheet zinc was hitherto ascribed to divergences in theproperties of the material, it being considered that the origin of thebrands of zinc used, the impurities, such as iron, lead, arsenic and thelike, affected the suitability of the metal for the purposes inquestion. It has also been considered that the superficial structure ofthe plate, the compactness and the smoothness of the surface might beinfluential factors, though no one was able to say what the character ofthe plate ought to be, from this point of view, and what would be theeffect of the various properties.

A specialist in these matters alleged that the Cohen test is the mostsensitve for cell plates, and that plates which stood that test aresuitable, in all circumstances, for articles of the highest quality. Itis, however, not impossible that plates which do not fulfill theconditions of the Cohen test will occasionally give good results, butwhen such plates are used, the quality of the articles is entirely amatter of chance.

(Cl. 148-1l.-5)

It has now been ascertained that rolled sheet zinc will aways not onlycorrespond to the Cohen conditions but will also far exceed the qualityhitherto attained, according to the Cohen test, if the basal surfaces ofthe (hexagonal) zinc crystals in the plate lie parallel with the surfaceof the latter. Plates exhibiting this structure can be produced indifferent ways. For example,'the niicrostructiu'e prescribed accordingto the invention, can be obtained from sheets which have been rolledfrom the ordinary cast zinc slabs, if they are annealed at temperaturesabove 100 C., for example between 200 and 400, followed by pressuretreatment, preferably by cold rolling to the desired gauge. this mannercan always be relied on to pass the Cohen test.

Still better results are obtained when the casting of the zinc-slabs isconducted in such a manner that the zinc crystals are disposed quiteirregularly in the slabs. In contrast to the general casting process,this irregular disposition of the zinc crystals in the cast slab, isobtained by pouring the molten zinc at a temperature slightly exceedingthe melting point, for example between 420 and 440 and preferably 430 C.This pouring temperature can be attained in a great variety of ways, forexample by cooling molten zinc, that has been heated to a highertemperature, until it begins to solidify, or the production of thispouring temperature may be effected directly in heat- Zinc platestreated in iii) ing the zinc placed in the melting furnace. For

example, the furnace containing the charge of zinc may first be heatedup until the zinc begins to melt, the fire being then drawn and the furnace left to itself for a short time, until the heat accumulated in themasonry and the like has assisted the fusion of the zinc to such anextent that 10-20% of the zinc charge can be scooped out of the furnace.The molten zinc is cast into slabs, preferably 10-11 mm. thick andweighing, when of the usual length and breadth, for example 17-18 kg.The fire is then ignited again, and working of the furnace and thepouring are resumed after suitable replenishment of the zinc.

This method of casting furnishes slabs consisting of very fine andirregularly disposed crystals, whereas in the usual casting process, allthe zinc crystals are oriented to a certain extent. Since the basalsurface of the hexagonal zinc crystal is a cleavage plane, theirregularly disposed small zinc crystals in these slabs arrangethemselves, in the course of rolling, so that the basal surfaces lie onor parallel with the surface of the plates.

Under the Cohen test, plates produced in this manner give a rise intemperature of 1-2 C. in 30 minutes, and do not reach 30 C. until after-110 minutes. Thus, the zinc plates produced in accordance with theinvention give substantially better values, in the Cohen test, than theplates hitherto employed for galvanic purposes which, although they maypass the test to some extent, do not have the structure of the plates ofthe invention. Moreover-and his constitutes the greatest technicaladvantage of the plates according to the invention, all the plates whichexhibit the structure according to the invention, invariably fulfil theconditions of the test, whereas in the case of the previous zinc plates,although, in some instances convertible into cell casings of highquality, there was no certainty that plates possessing these goodproperties could be produced, and no guarantee could be given as totheir quality. The orientation of the zinc crystals in the plate can beexamined in accordance with known physical methods.

If it be desired to improve still further the plates produced from theslabs according to the invention, this can be done by annealing theplates at the temperature already specified, followed by cold working.

The zinc plates according to the invention are suitable, not only forgalvanic purposes, but can also be employed with advantage in all caseswhere importance is attached to increased resistance of the zinc platesto corrosion.

I claim:-

1. A method for the production of zinc sheets particularly adapted foruse in galvanic cells which comprises casting relatively thin zincplates at a temperature of from about 420-l40 0., rolling said platesinto sheets in the usual manner, and thereafter cold rolling the sheets.

2. A method for the production of zinc sheets particularly adapted foruse in galvanic cells which comprises casting relatively thin zincplates from the molten portion of a charge of Zinc at a temperature offrom about 420 to 430 C. containing a substantial proportion of unmoltenzinc, and rolling said plates into sheets in the usual manner.

3. A method for the production of zinc sheets particularly adapted foruse in galvanic cells which comprises casting relatively thin zincplates from the molten portion of a charge of zinc at a temperature offrom about 420 to 430 C. containing a substantial proportion of unmoltenZinc, rolling said plates into sheets in the usual manner and thereaftercold rolling the sheets.

4. A method for the production of zinc sheets particularly adapted foruse in galvanic cells which comprises casting zinc plates at atemperature of from about 420 to 430 C. and rolling said plates intosheets in the usual manner.

5. A method for the production of zinc sheets particularly adapted foruse in galvanic cells which comprises casting zinc plates of not morethan about 15 mm. thickness at a temperature of from about 420 to e30 C.and rolling said plates into sheets in the usual manner.

6. A method for the production of zinc sheets particularly adapted foruse in galvanic cells which comprises partially melting zinc, castingthe molten zinc into plates at a temperature of about 430 C. and rollingthe plates into sheets in the usual manner.

PAUL MITTMANN.

